1. Field of the Invention
This invention pertains generally to field-effect transistors (FETs) and in particular to junction FETs made by ion implantation from III-V semiconductors.
2. Prior Art
Present-day semiconductor electronics is dominated by silicon technology. However, it is recognized that other semiconducting materials, though lacking silicon's technological maturity, offer the promise of improved speed and performance.
In a microwave FET, the velocity of the charge carriers and parastic elements play important roles in determining its high frequency performance. At low electric fields, charge carrier velocity is proportional to the electric field with the proportionality constant being the drift mobility. However, at higher electric fields the relationship becomes non linear. For Si, the drift velocity reaches a saturation value at fields above 5.times.10.sup.4 V/cm. For GaAs and InP, the drift velocity first reaches a peak and then declines to a saturation value. Since the low-field mobility of silicon is relatively small, the performance of GaAs and InP is expected to be superior to that of Si. InP is expected to operate at higher frequencies than GaAs due to its higher peak velocity. In addition, the higher reverse breakdown voltage and higher thermal conductivity of InP also gives it the potential for outperforming GaAs.
Early device designers, faced with the choice of GaAs, InP and other III-V compounds besides GaAs and InP, picked GaAs because of its perceived technological advantages. The primary FET structure for GaAs is the metal-semiconductor field-effect transistor (MESFET). MESFETs made from GaAs are relatively easy to fabricate because of the large Schottky barrier between the metal and the semiconductor. However, the Schottky barrier for InP and III-V alloys such as InGaAs is too small to easily fabricate Schottky barrier FETs. Thus InP and InGaAs were not extensively pursued for use in FETs.
A junction gate structure in InP circumvents the Schottky barrier problem and early work on diffused JFETs (surface diffusion using Zn dopants) have been reported. Ion implantation of the dopants is an attractive alternative approach since it is highly compatible with the fabrication of planar devices and monolithic circuits. Ion implantation has been used to make FETs from InP but these efforts have been limited to MESFETs. The inventor participated in research leading to the present invention that resulted in a doubly ion implanted InP JFET. However, this device needed to be isolated by an etch which created a mesa structure on which it was difficult to deposit interconnect lines.